An important consideration in the design of an optical processor for telecommunications applications is minimization of the processor's package size. For example, because optical processors are commonly installed in a conventional card-in-rack manner, the package thickness of a given processor may determine whether the processor will fit in a rack, and the package length and width will determine how much space the processor will take up on a card. However, while package size is a significant concern, any design must be made with consideration for optical performance (e.g., aberration correction and light throughput).
A particular type of telecommunications processor having a relatively large package size is a demultiplexer of optical carriers of a wavelength-division multiplexed (WDM) signal. Demultiplexers are commonly used as a part of more complex processors to achieve spatial separation between optical carriers so that the optical carriers can be processed individually. For example, demultiplexers may be used in gain equalization filters (GEFs) and in optical adding and dropping modules (OADMs).
One option for reducing the package size of a demultiplexer is to incorporate the demultiplexer into a fiber, such as by using a fiber Bragg grating (FBG). While fiber grating structures can achieve demultiplexing in a compact space, they exhibit performance limitations such as temperature sensitivity. Additionally, because the number of carriers that can be demultiplexed by an FBG is proportional to the length of the FBG, as the number of optical carriers included in conventional wavelength-division multiplexed signals increases, the size advantages provided by FBGs is being reduced.
Free-space optics provide an alternative to FBGs. Free-space demultiplexers typically include a diffraction grating to achieve spatial separation of optical carriers. The diffraction gratings angularly separate the optical carriers, such that the angle of emergence of the carriers from the grating is a function of wavelength. Following angular separation, the angularly separated optical carriers are propagated to allow the optical carriers to separate spatially. Accordingly, the package size of a free-space demultiplexer is at least partially determined by the spectral separation of the carriers to be demultiplexed and the amount of spatial separation to be achieved. The package size required to achieve adequate spatial separation provided by conventional free-space demultiplexers is becoming larger as the spectral separation between the optical carriers comprising WDM signals is becoming smaller.
FIG. 1 is a schematic of a conventional optical processor 100 conducive to the reduction of package size. In FIG. 1, a fiber 110 provides a wavelength-division multiplexed signal 105 to a conventional demultiplexer 170, including a collimating lens 120, a grating 130, and a focusing lens 140 (having a focal length f2). Demultiplexer 170 spatially separates the component optical carriers 125A and 125B of the WDM signal, and focuses the spatially-separated optical carriers onto a reflective optical modulator 150.
More specifically, the WDM signal is collimated by lens 120 to form a beam 122, the optical carriers comprising the WDM signal are angularly separated by grating 130, such that each separated optical carrier forms a collimated beam (e.g., beams 126A, 126B), and each such collimated beam propagates at an angle with respect to optical axis 160 of lens 140. Optical axis 160 extends in the direction of the x-axis.
Lens 140 is telecentricly located (i.e., lens 140 is located one focal length f2 from both grating 130 and modulator 150), such that it focuses optical carriers 125A and 125B and projects each of them in a cone of light having a respective centerline 127A, 127B. Each centerline 127A, 127B is normal to a surface of a respective pixel 135A, 135B of modulator 150. Each optical carrier 125A, 125B is modulated by actuation of one or more operational components of the pixels 135A and 135B. The components of demultiplexer 170 (i.e., collimating lens 120 grating 130 and positive lens 140) are selected to achieve an amount of spatial separation between carriers 125A and 125B in the y-direction on modulator 150. Accordingly, the operative elements 135A and 135B can modulate individual optical carriers 125A and 125B.
Because lens 140 projects each optical carrier in a cone of light having a centerline 127A, 127B normal to a surface of reflective optical modulator 150, optical carriers 125A and 125B can be re-multiplexed by transmitting a reflected portion of the modulated optical carriers back through demultiplexer 170 in the reverse direction of the unmodulated carriers.
Although the processor of FIG. 1 provides a relatively compact package size compared to prior designs, the processor remains relatively large. That is, for a processor 100 to achieve adequate spatial separation of optical carriers 125A and 125B, the dimensions of optical processor 100 measured in the x-direction (i.e., package length) and in the y-direction (i.e., package width) remain relatively large. For example, a conventional optical processor may have a length of 200 mm, a width of 100 mm and a thickness of 20 mm. Accordingly, there remains a need to reduce the package size of multiplexer/demultiplexers.